The present invention relates to a toner for developing an electrostatic latent image which is used for developing an electrostatic latent image in electro-photography, electrostatic recording, etc. Particularly, it relates to a toner for developing an electrostatic latent image which is good in ease of cleaning, namely, which can easily be removed from the surface of an image carrier after transfer.
In conventional image-forming methods utilizing electrophotography or electrostatic recording, an electro-static latent image formed on the surface of an image carrier composed of a photoconductor, a dielectric or the like is developed into a toner image by brushing with a so-called magnetic brush composed of a developer, by the use of, for example, a developing means comprising a permanent magnet member built-in and a sleeve fitted thereto so that they can rotate freely in relation to each other. Then, the toner image is fixed directly or after its transfer to a transfer sheet such as ordinary paper, whereby a final image is obtained.
As a conventional process for producing a toner constituting the aforesaid developer, there is known a process of heating raw materials, followed by kneading, solidifying by cooling, grinding and then classification. For obtaining an image having a high solid density and little fog, the particle size distribution of the toner is usually adjusted so that the average particle size may be 9 to 13 .mu.m. However, recently, there is a growing demand for improvement of the quality of image. For satisfying this demand, the toner tends to be reduced in particle size and should be formed so as to have an average particle size of 2 to 9 .mu.m.
The toner with a small particle size produced by the above process involving grinding, however, is disadvantageous in that it has a low flowability because its particles are shapeless. When a large amount of a flowability improver such as finely powdered silica is added for improving the flowability, the flowability is improved but a problem is caused such as damage to the surface of a photosensitive layer, or a great change of the degree of tribo-electric charging which is attributable to moisture.
In order to solve the problems in the above process involving kneading and grinding, processes for producing a toner by suspension polymerization have been proposed (see, for example, Japanese Patent Unexamined Publication Nos. 54-84730, 56-110947 and 59-28165). In these processes comprising suspension polymerization, a monomer composition prepared by dissolving or dispersing a polymerizable monomer, a colorant, and optionally a polymerization initiator, a crosslinking agent, a charge-controlling agent and other additives is added to a dispersion medium containing a suspension stabilizer with stirring to be granulated, followed by polymerization, whereby a toner is formed.
Since the above processes comprising suspension polymerization do not involves a grinding step at all, the product need not be brittle or fragile, and the colorant and the like do not appear at the rupture cross-section of the toner formed by grinding. Therefore, these processes are preferable production processes. Moreover, they are considered advantageous, for example, in that particles of the toner obtained has a spherical shape and hence are excellent in flowability.
As a method for forming an image by the use of a two component magnetic developer containing a toner such as is described above, a method comprising development, transfer, fixing and then fur brush cleaning has been made practicable. For obtaining a highly precise image, employment of a toner with a small particle size, such as is described above is effective, and it is important to remove the excess toner adhering to the surface of an image carrier after transfer.
The toner produced by the above process comprising suspension polymerization, however, is disadvantageous in that it has a particle size (average particle size: 10 to 11 .mu.m) smaller than that (average particle size: 2 to 9 .mu.m) of a usually used toner and hence is difficult to remove from the surface of the image carrier after transfer, namely, it is poor in so-called "ease of cleaning". That is, the excess toner remains on the surface of the image carrier even after the fur brush cleaning and is transferred to an undesired area to deteriorate the quality of image.